You may think you live on a planet, but really you live on a gigantic farm, one occasionally broken up by cities, forests and the oceans. Some 40% of the world’s land surface is used for the purposes of keeping all 7 billion of us fed — albeit some of us, of course, more than others. And the vast majority of that land — about 30% of the word’s total ice-free surface — is used not to raise grains, fruits and vegetables that are directly fed to human beings, but to support the chickens, pigs and cattle that we eventually eat.

Livestock production — which includes meat, milk and eggs — contributes 40% of global agricultural gross domestic product, provides income for more than 1.3 billion people and uses one-third of the world’s fresh water. There may be no other single human activity that has a bigger impact on the planet than the raising of livestock. But as a new study out today in the journal Proceedings of the National Academy of Sciences (PNAS) shows, there is tremendous variation in how we raise livestock around the world — and major differences in what that means for the earth and for us.

Researchers from the International Livestock Research Institute in Kenya, the Commonwealth Scientific and Industrial Research Organization (CSIRO) in Australia and the International Institute for Applied Systems Analysis (IIASA) in Austria produced a comprehensive assessment of the livestock industry around the world, in developed nations where factory farming is common and in developing nations where livestock are more likely to graze on grasslands. They dug up some striking statistics that underscore just how much meat production varies from region to region.

Each year the livestock sector globally produces 586 million tons of milk, 124 million tons of poultry, 91 million tons of pork, 59 million tons of cattle and buffalo meat, and 11 million tons of meat from sheep and goats. That 285 million tons of meat altogether — or about 36 kg (80 lb.) per person, if it were all divided evenly. It’s not — Americans eat 122 kg (270 lb.) of meat a year on average, while Bangladeshis eat 1.8 kg (4 lb).

Of the 95 million tons of beef produced in the world in 2000, the vast majority came from cattle in Latin America, Europe and North America. All of sub-Saharan Africa — a region with nearly three times as many people as the entire U.S. — produced just 3 million tons of beef.

1.3 billion tons of grain are consumed by farm animals each year — and nearly all of it is fed to livestock, mostly pork and poultry, in the developed world and in China and Latin America. All of the livestock in sub-Saharan Africa eat just 50 million tons of grain a year, otherwise subsisting on grasses and on crop residue.

The poor feed quality in impoverished regions like sub-Saharan Africa means that a cow there may consume as much as 10 times more feed — mostly grasses — to produce a kilogram of protein than a cow raised in richer regions. That lack of efficiency also means that cattle in countries like Ethiopia and Somalia account for as much as 1,000 kg of carbon for every kg of protein they produce — in the form of methane from manure as well as from the reduced carbon absorption that results when forests are converted to pastureland. That’s 10 times higher than the amount of carbon released per kg of protein in many parts of the U.S. and Europe, where livestock production is much more intensive.

About that: in North America or Europe, a cow consumes about 75 kg to 300 kg of dry matter — grass or grain — to produce a kg of protein. In sub-Saharan Africa, a cow might need 500 kg to 2,000 kg of dry matter to produce a kg of protein, because of the poor feed quality in arid countries and because of the high mortality rates in herds of often undernourished and sick animals.

The highest total of livestock-related greenhouse-gas emissions comes from the developing world, which accounts for 75% of the global emissions from cattle and other ruminants and 56% of the global emissions from poultry and pigs.

The most climate-friendly meats comes from pigs and poultry, which account for only 10% of total livestock greenhouse-gas emissions while contributing more than three times as much meat globally as cattle. Pork and poultry are also more efficient for feed, requiring up to five times less feed to produce a kg of protein than a cow, a sheep or a goat.

So what does this all mean? While factory farming in the U.S. gets a lot of criticism for its cruelty, the danger it poses to public health through the overuse of antibiotics and the pollution it causes to air and water, it can be remarkably efficient. And given the fact that the planet isn’t getting any bigger while the global population and the global appetite keep growing, efficiency is going to matter when it comes to food production. The upside of inefficient livestock production in the developing world is that there is a lot of room to improve, given the right kind of help — which is exactly what the authors of the PNAS paper are hoping for.

“Our data can allow us to see more clearly where we can work with livestock keepers to improve animal diets so they can produce more protein with better feed while simultaneously reducing emissions,” said Petr Havlik, a research scholar at IIASA and a co-author of the study. What we need is “sustainable intensification” — efficiency but pursued in a measured way.

That’s not to say it would be advisable simply to export developed-world livestock practices to, say, desperately poor, climatically challenged countries, even if it were possible. The low livestock-feed efficiencies in sub-Saharan Africa is due to the fact that most animals in the region consist on vegetation that is not edible by human beings — a fact that’s fairly important in a region where grain is simply too precious to use for animals. Livestock also serves a different function in the developing world. “Cattle and poultry can be walking banks in the developing world,” says Mario Herrero, an agricultural-systems scientist at CSIRO and a co-author of the paper. “They provide manure to small-holder farmers. There’s a tremendous social role for livestock that can’t be ignored.”

Above all else, the study underscores that while meat production will need to change in the future, so will meat consumption. It’s difficult to get a full and proper accounting of the total environmental impact of livestock production. A 2006 report from the Food and Agriculture Organization estimated that livestock were responsible for about 18% of human-caused greenhouse gases — a figure that has been criticized by the meat industry as too high and by some environmentalists as far too low. But what’s clear is that American levels of meat consumption can’t be sustainably adopted by the rest of the world, even if livestock management becomes more efficient globally. “Demand management has to be part of the solution as well,” says Herrero. For the environment — and for our hearts and waistlines too.